Diagnosing The Flu

Your respiratory illness might be the flu if you have fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and fatigue. Some people may have vomiting and diarrhea. People may be infected with the flu and have respiratory symptoms without a fever. Flu viruses usually cause the most illness during the colder months of the year. However, influenza can also occur outside of the typical flu season. In addition, other viruses can also cause respiratory illness similar to the flu. So, it is impossible to tell for sure if you have the flu based on symptoms alone. If your doctor needs to know for sure whether you have the flu, there are laboratory tests that can be done.

What kinds of flu tests are there?

A number of flu tests are available to detect influenza viruses in respiratory specimens. The most common are called “rapid influenza diagnostic tests (RIDTs).” RIDTs work by detecting the parts of the virus (antigens) that stimulate an immune response. These tests can provide results within approximately 10-15 minutes, but are not as accurate as other flu tests. Therefore, you could still have the flu, even though your rapid test result is negative. Other flu tests are called “rapid molecular assays” that detect genetic material of the virus. Rapid molecular assays produce results in 15-20 minutes and are more accurate than RIDTs. In addition, there are several more-accurate and sensitive flu tests available that must be performed in specialized laboratories, such as those found in hospitals or state public health laboratories. All of these tests require that a health care provider swipe the inside of your nose or the back of your throat with a swab and then send the swab for testing. Results may take one hour or several hours.

How well can rapid tests detect the flu?

During an influenza outbreak, a positive rapid flu test is likely to indicate influenza infection. However, rapid tests vary in their ability to detect flu viruses, depending on the type of rapid test used, and on the type of flu viruses circulating. Also, rapid tests appear to be better at detecting flu in children than adults. This variation in ability to detect viruses can result in some people who are infected with the flu having a negative rapid test result. (This situation is called a false negative test result.) Despite a negative rapid test result, your health care provider may diagnose you with flu based on your symptoms and their clinical judgment.

Will my health care provider test me for flu if I have flu-like symptoms?

Not necessarily. Most people with flu symptoms do not require testing because the test results usually do not change how you are treated.

Your health care provider may diagnose you with flu based on your symptoms and their clinical judgment or they may choose to use an influenza diagnostic test. During an outbreak of respiratory illness, testing for flu can help determine if flu viruses are the cause of the outbreak. Flu testing can also be helpful for some people with suspected flu who are pregnant or have a weakened immune system, and for whom a diagnosis of flu can help their doctor make decisions about their care.